The invention relates generally to a strain measuring device and more specifically concerns a device for measuring changes in strain of any materials (metals, composites, glass, rock, concrete, etc.) having many geometries (bolts, rods, plates, blocks, in situ mine shafts).
The prior art for determining fastener tension using torque depends on the repeatability and consistence of friction--a fact which is not achievable. Even for critical fasteners with extreme process control and care in storage and application, torque is capable of at best a nominal 15% variation in fastener strain with a given torque. In most applications, the errors obtained using torque are far greater due to the fact that up to 90% of the applied torque overcomes friction--only 10% goes into fastener strain. Thus a variation of 10% in friction can cause a 100% change in strain.
Other techniques, which are accurate, measure fastener elongation or direct strain. The simplist of these methods include micrometer measurements which are slow, require access to both sides of the fastener or a knowledge of the joint compliance and therefore are impractical. Electronic strain gaged bolts are costly and require highly stable bonds of the strain gage to the fastener for accuracy.
Ultrasonic techniques have been developed for bolt tension measurements which use shock excited acoustic pulse time of flight measurements or multiple pulse overlap/null methods or continuous wave high harmonic resonance techniques.
The shock excited methods are inherently broadband and thus launch an acoustic wave containing many frequencies. Each frequency propagates with a unique diffraction producing many different effective path lengths in the group velocity waveform. Also, this technique requires high speed electronics to achieve the accuracy necessary for bolt tension measurements (typically nanosecond resolution for a 10 cm bolt of round trip time 35.times.10.sup.-6 sec or 3 parts in 10.sup.4).
The double pulse overlap method is difficult to automate and requires considerable judgment on the part of the user. Furthermore, this method is unnecessarily complicated and can easily be misread when cycle for cycle overlap is shifted by 2.pi..
A gated RF tone burst technique has been disclosed which requires user judgment to achieve a proper phase locking point and an inability to maintain that locking point under sample changes without user input. In addition, this device does not take advantage of instantaneous phase measurement, but rather averages the phase signal over several microseconds. This produces greater instability for geometrically varying situations such as occur in a fastener under preloading. Furthermore, the time delay involved with using the integral of the sample during the phase gate does not allow use of layered samples with shifts occuring during the initial pulse propagation phase gate window.
A continuous wave (CW) method which depends on resonance techniques requires high Q fastener geometry to achieve an acoustic spectra free of structure which may cause locking errors. As the bolt geometry becomes nonideal, as may occur in a fastener with a hole, markings on its surface or rough nonparallel surfaces, the CW approach becomes impractical.
It is an object of this invention to provide a device for measuring changes in strain of any material having many different geometries.
Another object of this invention is to provide a device for measuring changes in strain in which no user judgment is necessary.
A further object of this invention is to provide a device for measuring strain that takes advantage of instantaneous phase measurements.
Still another object of this invention is to provide a device for measuring strain which is not easily misread when cycle for cycle overlap is shifted by 2.pi..
A still further object of this invention is to provide a device for measuring the strain in a sample whose geometry is nonideal.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.